The histone fold forms the fundamental endoskeleton of the protein core of the nucleosome and is also found in several transcription factors. We have investigated the evolutionary origins of this ubiquitous protein motif, which is found soluble exclusively as an antiparallel (handshake motif) dimer. We introduced a three amino acid insertion into the middle of a homodimeric archaeal histone fold motif. The engineered molecule was found to be a soluble and stable monomer with properties consistent with a four-helix-bundle protein. The experimental evidence presented here support the hypothesis that the handshake association motif characteristic of present-day histone dimers is the evolutionary product of domain swapping between two four-helix bundle domains, each of which derived from the tandem duplication of a primitive helix-strand-helix unit.